If you're developing an application with a Windows desktop GUI, you're still doing it on full framework (WPF, WinForms, UWP). They're folding that into .NET 5, and I haven't seen anything indicating that they see that as "doing it wrong". This is only one case of developing for full framework, but it's a big one. I imagine any other cases where the developer community really needs full framework capabilities that still aren't ported to CoreCLR, they'll fold that in just the same as with the UI frameworks, as a Windows-specific capability. Maybe they end up sunsetting more than I think, but for now this seems more like consolidating Core and Framework than ending Framework.
Fair point if the problems are just crashes, but I wouldn't be surprised if Belichick was getting that frustrated over much more minor things at a time when the game is not on the line. In my experience many older people have extremely low tolerance for usage of technology not going perfectly.
For example, I could picture BB chucking the tablet from something like a slight lag when navigating menus or even just dissonance between how the app works and how he expects it to work (regardless whether or not poor design is to blame).